The U.S. dollar has gained strength in recent weeks, but that may change in the second half of the year, according to a portfolio manager from Conning Asia Pacific.

The dollar index, which measures the greenback against six major currencies, climbed 3.43 percent in the last two weeks as U.S. economic data has been better than those out of the eurozone and Japan. That climb has reversed the general downtrend the currency saw over 2017.

But dollar strength is likely to last only "a fairly finite period of time" as the eurozone and Japan catch up in the second half of 2018, Marc Franklin, a senior portfolio manager at Conning Asia Pacific, said on Wednesday.

"Structurally, there's going to a growing supply of dollars in the system over the next two to three years, and that potentially creates a structural headwind for the dollar, longer term."-Marc Franklin, portfolio manager, Conning Asia Pacific

"What's happened most recently has been a growing divergence in the growth in inflation data in the U.S. versus Europe and Japan, in particular," Franklin told CNBC's "Capital Connection."

"But as you start to go into the second half of this year, eurozone inflation — assuming oil prices stay where they are — starts to tick up, as does Japanese inflation as well," he said.

The twin deficits in the United States are seen worsening, thanks to President Donald Trump's tax cuts. That's not helping the prospects of the greenback, Franklin said.

A widening current account deficit and larger government budget increase the supply of U.S. dollars over time, he explained.

"Structurally, there's going to a growing supply of dollars in the system over the next two to three years, and that potentially creates a structural headwind for the dollar, longer term," he said.